2 December 2013

An evaluation of different fuel cell hybrid electric vehicle (FCHEV) powertrain designs—such as fuel cell/supercapacitor (FC/SC), fuel cell/battery (FC/B), and a combination of supercapacitors and batteries (FC/SC/B)—and different control strategies by researchers in Belgium concluded that the FC/SC HEV has slightly higher fuel economy than the FC/B HEV and FC/B/SC HEV powertrains.

This, the researchers suggested in a paper presented at the recent EVS 27 conference in Barcelona, was due to the use of the efficient supercapacitors for the majority of the transient-power requirements (the SC can be charged or discharged at a high current, at which the battery cannot function). The fuel economy of the supercapacitor fuel cell hybrid, they noted, is higher despite the vehicle being heavier and more expensive.

They also pointed out that the combination of supercapacitors and batteries (FC/B/SC HEV) may provide a good solution for FCHEVs from the point of view of battery lifespan, component sizing and transient periods.

A stand-alone FC [fuel cell] system integrated into an automotive powertrain is not always sufficient to satisfy the load demands of a vehicle. Although FC systems exhibit good power capability during steady-state operation, the response of fuel cells during transient and instantaneous peak power demands is relatively poor. Consequently, the high cost and slow dynamics of the FC systems are the major challenges for the commercialization of fuel cell electric vehicles (FCEVs).

To overcome these challenges, the FC system should be hybridized with single or multiple energy storage systems (ESS) (such as battery and supercapacitor) to meet the total power demand of a hybrid electric vehicle (HEV) and to improve the efficiency.

… The main objective of this paper is to give an evaluation study of different FCHEV powertrains from the point of view of the fuel economy, cost and powertrain component sizing.

—Hegazy et al.

In the study, they designed and simulated the different fuel cell hybrid powertrains using Matlab/Simulink; two standard driving cycles (NEDC and FTP75) were used to evaluate the fuel consumption.Further, two control strategies based on the knowledge of the fuel cell efficiency map were implemented to minimize the hydrogen consumption of the FCHEV powertrains.

These control strategies were (1) a control strategy based on the Efficiency Map (CSEM); and (2) and control strategy based on Particle Swarm Optimization (CSPSO). CSEM is applied to minimize the hydrogen consumption for each driving cycle. CSPO instantaneously distributes the power between the multiple sources with the aim to minimize the hydrogen consumption while maintaining the SoC of
the ESS over the driving cycle.

Comparison of the improvement in hydrogen fuel economy between the FCHEV powertrains over different driving cycles. Left: CSEM. Right: CSPSO. Click to enlarge.

Comments

Sounds an ideal use for the graphene supercapacitors being developed in Korea:

''They say it has a specific capacitance of over 150 Farrads per gram can store energy at a density of more than 64 watt-hours per kilogram at a current density of 5 amps per gram.

That’s almost comparable with lithium-ion batteries, which have an energy density of between 100 and 200 watt-hours per kilogram.'
http://www.technologyreview.com/view/521651/graphene-supercapacitors-ready-for-electric-vehicle-energy-storage-say-korean-engineers/

Presumably the Hyundai engineers leading the way in fuel cell cars are all over this.

The power density of those graphene supercaps is surprisingly low. 64 Wh/kg = 216 J/g; at 150 F/g, I get a peak voltage of 1.7 volts, holding 250-odd coulombs per gram peak. 5 A/g is 1/50 of full charge per second. Even figuring that the useful charge range only goes down to half, that's 25 seconds,

I'm waiting for the superflywheels to come along to compete with the supercaps. They're already proven in racing, it's a question of how practical they can be made and how cheaply.

Superflywheels and supercaps are ideal to recover braking energy and for almost unlimited number (1,000,000+) of short (30 seconds or so) burst of energy. That would be enough for normal accelerations but not the best for long hill climbing.

It may or may not be easier to fit and control half a dozen supercaps into a small car?

Please pack my future hydrogen car with all the necessary amenuities like super caps, batteries, software, rectifiers, connectors, efficient compact fuelcell, tanks, valves, carbon fibers, etc. Lets do this in a modern state of the art way at a good price. If not i will buy a used small manual gasoline car like the one i have right now. A 2016 used gasoline ford fiesta 1 liter that i might buy in 2023 can do the trick.

They don;t say how much each of the 3 scenarios weigh or cost.
I wonder why the FC-SC combo is better than the FC-SC-Ba combo - is it the extra weight of the battery ?
+
all this is a simulation - the real world might be a bit different.
+
What does this suggest for ICE hybrids - would they be better with SCs or batteries (or both), and at what cost.
What is the minimum amount of sc that you need to have a big impact - do you need to be able to accelerate a car from 0-30/40/50/60 mph ? [ my guess is about 40 ]

Why is the FC-SC combo better than FC-Ba and FC-SC-Ba?
This is due to the absent of internal resistance of the SC, in comparison to battery. Let's compare this to the Prius battery: 1300 Wh weighing 41 kg, or 32 Wh/kg, but is capable of only 21 kW in Prius Gen II and 27 kW in Gen III, or 0.5 to 0.65 kW/kg. THe Prius' battery only uses up to 40% of its capacity to preserve battery life, so only 520 Wh of energy maximum is deposited or removed from the battery. The battery is never charged above 70% and never drained below 30%. At high rates of current flow, batteries' efficiency decrease significantly, and significantly heat up the battery, requiring active cooling.

By contrast, a SC has no internal resistance, so the power capacity is only limited by the power of the inverter, which is ~100kW in a typical FCV. No loss of energy due to having no resistance within the SC therefore efficiency increase. Plus, a SC can be charged to near 100% and drained until near empty without impacting lifespan, so, if 520 Wh is need for power buffering, only ~600-800 Wh of capacity of SC is needed. So, with specific energy of 64 Wh/kg, only ~9 kg of SC will be needed, not the 41 kg of battery weight like in the Prius.

Battery will age significantly within 5-10 yrs regardless of use, faster with higher ambient temperatures. SC will last longer, 10-15 years or so and thus more cost effective in the long run, with nearly unlimited cycle life.

However, typical specific energy of SC commercially-available at the present is in the low tens of Wh/kg, and this is still perfectly OK.

That is a LOT of material (you can cut it somewhat by not using the bottom of the voltage range, but that cuts your power surge duration). It's as much as a hybrid battery. And it doesn't include packaging or connections.

I bet that the FC came out on top because the stack size was reduced a lot more by off-loading the peak power demand to the supercap than a battery pack was, and regeneration improved (may have been the opposite on a highway driving cycle). There's no link to the report so I can't judge right away about the assumptions made for the supply chain.

The positive side of the new higher performance supercaps is that they:

1. weight 10 times less than before and improving.
2. can be charged and discharges very quickly for 1,000,000+ times.
3. can recover more braking energy than batteries.
4. ideal for stop and go FC vehicles of all size.
5. will weight less and less year after year with new technologies.

The price per kWh is currently higher than batteries but mass production could make them competitive?

@EP:
I think you have dropped your decimal places.
You only need the 150kw for an acceleration event for 20 seconds or so.
Call it 30 seconds for easy calculation, and you have 1.25kwh of storage needed.

Actually from a dim memory of a discussion I had with a guy who was very knowledgeable on capacitors, I believe they they were looking at specifying around 300wh capacity for capacitor assist.
They were looking at small cars with nothing like 150kw on tap, and some of the power comes from the normal battery use, just topped up by the capacitors.

Davemart: at 5 A/g, the numbers work out as I wrote them. 64 Wh/kg is 230.4 J/g. Energy in a capacitor is ½CV². At 150 F/g, this comes out to 1.75 volts max and about 260 coulombs per gram from zero to full charge. That's 52 seconds to charge or discharge the thing.

@EP:
Not being an engineer I can't follow the figures you give in detail.
However the Peugeot's use nothing remotely like 35kg of capacitors, and they are looking at putting less than 1kwh of capcitors into projects.

Supercapacitors is a good idea, but using them for fuel cell cars will not make the fuel cells cars as good as cars that do not involve the extra step of making H2 hydrogen from otherwise good electricity.

Our old sun and distributed wind can supply more than enough 'good electricity' at reasonable cost in the near future. Using some of it to cleanly produce H2 for FC vehicles and for rainy days may not be such a bad idea.

Safe, affordable nuclear plants may be another possibility by 2050+ but not in the short term.